In order to identify the main social policy tools that can efficiently combat working poverty, it is essential to identify its main driving factors. More importantly, this work shows that all poverty factors identified in the literature have a direct bearing on working households through three mechanisms, namely being badly paid, having a below-average workforce participation, and high needs.One of the main purposes of this work is to assess whether the policies put forward in the specialist literature as potentially efficient really work. This is done in two ways. A first empirical prong provides an evaluation of the employment and antipoverty effects of these instruments, based on a meta-analysis of four instruments: minimum wages, tax credits for working households, family cash benefits and childcare policies. The second prong relies on a broader framework based on welfare regimes. This work contributes to the identification of a typology of welfare regimes that is suitable for the analysis of working poverty, and four countries are chosen to exemplify each regime: the US, Sweden, Germany, and Spain. It then moves on to show that the weight of the three working poverty mechanisms varies widely from one welfare regime to the other. This second empirical contribution clearly shows that there is no "one-size-fits-all" approach to the fight against working poverty.But none of this is possible without having properly defined the phenomenon. Most of the literature is characterized by a "definitional chaos" that probably does more harm than good to social policy efforts. Hence, this book provides a conceptual reflection pleading for the use of a very encompassing definition of being in work. It shows that "the working poor" is too broad a category to be used for meaningful academic or policy discussion, and that a distinction must be operated between different categories of the working poor. Failing to acknowledge this prevents the design of an efficient policy mix.